Early geophysical papers of the Society of Exploration Geophysicists . earlygeophysical00soci Year: 1947 CALCULATION OF MOTION OF GROUND 87 Aw2 y = (cos ut — cos fit) m2 — n* we get when c is very small y= —A/it/2 sin/x/. If A = 1 andco=/i = 27r/12 this gives - -y = — t sin (30/)° 12 and x = 1 - cos (30/)°. Fig. 5 shows the curves for y in this case. The amplitude of the first oscillation is only about one half that of the ground but the amplitude rapidly increases as the successive waves arrive. Fig. 5. We see from this that for the purpose of detecting the arrival of the begin- ning of a
Early geophysical papers of the Society of Exploration Geophysicists . earlygeophysical00soci Year: 1947 CALCULATION OF MOTION OF GROUND 87 Aw2 y = (cos ut — cos fit) m2 — n* we get when c is very small y= —A/it/2 sin/x/. If A = 1 andco=/i = 27r/12 this gives - -y = — t sin (30/)° 12 and x = 1 - cos (30/)°. Fig. 5 shows the curves for y in this case. The amplitude of the first oscillation is only about one half that of the ground but the amplitude rapidly increases as the successive waves arrive. Fig. 5. We see from this that for the purpose of detecting the arrival of the begin- ning of a train of waves, it is not a good plan to use a seismograph having a free period equal to that of the waves. A seismograph with a longer period gives twice as great a deflection due to the first wave as one with a period equal to that of the waves. Of course at the start — y = x in all cases so that the initial sensibility is independent of the periods but it is advantageous to get a larger deflection due to the first wave since a large deflection is less likely to be overlooked or obliterated by the small oscillations due to local disturbances. Now consider the third case when the period of the seismograph is smaller than that of the waves. The equation Aco2 y = (cos nt — cos oj/) 231
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